According to the PDF/A-3 specification:
The keyword Encrypt shall not be present in the trailer dictionary.
NOTE 2 The explicit prohibition of the Encrypt keyword has the implicit effect of disallowing encryption and password-protected access permissions.
(ISO 19005-3, section 6.1.3 - File trailer)
(Similarly in ISO 19005-1 and ISO 19005-2, and surely also in ISO 19005-4.)
In other words, password protection is forbidden in PDF/A documents.
This is not really surprising; PDF/A is about making sure the content is and will remain accessible for a long time. Passwords counteract this purpose...
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…